Acoustic Position Reference Methods for Offshore Drilling Operations

ABSTRACT The increasing use of reliable and flexible position reference systems for offshore operations is placing added emphasis upon the use of acoustic position-reference systems. Position measurement requirements exist for both navigation and precise station-keeping applications. To meet these requirements, several acoustic position reference configurations are available. In this paper, the various acoustic methods and position reference configurations are described and their merits and limitations are discussed. For the offshore drilling operations the short-baseline configuration which measures the position of a single beacon or transponder in ship coordinates is a superior choice. The short-baseline systems are more flexible, more convenient to operate, less complex, and less expensive to operate than long-baseline systems. The latter establish the position of the vessel relative to a transponder grid mounted on the ocean floor. Without a precise sound-velocity-profile calibration and correction, long-baseline systems show no improvement in accuracy over the short-baseline systems. For stat ion-keeping operations the Short-Baseline Configuration is recommended. This configuration can be implemented with a null accuracy, over the beacon, of 0.5 percent of water depth. For a navigation application, the short-baseli.ne beacon configuration does not have the range capability. However, the short-baseline transponder configuration, which measures its position from a range-bearing method, can adequately provide for the navigation requirements. It is concluded from the standpoint of equipment complexity, operating convenience, and operating cost that the short-baseline transponder configuration provides a superior choice over the long-baseline configurations. For applications such as self-anchor laying or pipe-laying navigation, the short-baseline transponder system can adequately provide the need. INTRODUCTION In the expanding offshore drilling operations the requirements for reliable position references are becoming exceedingly more important. With drilling operations moving to deeper waters, the submersible rigs (resting on the bottom) are unattractive and, therefore, floating rigs, semisubmersibles, barges and ships are used. These vehicles must be either moored (anchor net), actively positioned, or both. In each case, the necessity for an accurate position reference exists. Once the desired drilling location has been established (usually by radio navigation), it is necessary to maintain position reference relative to this location. Radio navigation, although sufficiently accurate for general navigational purposes, does not usually possess the accuracy and resolution necessary for precise station-keeping operation. Taut-wire systems are also used as position reference sources. These, however, lack in flexibility, accuracy and operating convenience. Position reference systems based upon acoustic methods can adequately provide for the position reference needs in offshore operations. A number of methods are available, each displaying its own set of advantages and disadvantages. This paper provides a description of the basic principles of operations, the implementation and the performance capability of each candidate acoustic position reference system. Several candidate systems are described and evaluated relative to the requirements in offshore operations, station keeping and limited range navigation. ACOUSTIC METHODS In acoustics a source (projector) sends acoustic energy into the medium and the energy propagates through the medium as explained by the complex wave propagation equations.